Response of corneal hysteresis and central corneal thickness following clear corneal cataract surgery

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  • IntroductionCorneal hysteresis (CH) and centralcorneal thickness (CCT) are importantfactors reecting the biomechanicalproperties of the cornea (Kotecha2007; Luce 2005). Whereas CH pre-dominantly reects the viscous proper-ties of the cornea (Kotecha 2007), theCCT parameter is correlated with cor-neal elasticity (Kotecha 2007; Peposeet al. 2007). Until recent years, CCTwas the only factor that could be mea-sured easily in vivo. With the innova-tion of ocular response analyzer(ORA) (Reichert Ophthalmic Instru-ments, Buffalo, NY, USA), a newmetric, CH was dened and its inu-ence needs further evaluation.Recent studies (Hager et al. 2007a;

    Kucumen et al. 2008) have demon-strated a reduction in CH in the imme-diate postoperative period followingclear corneal cataract surgery. In addi-tion, the swelling effect of corneaexposed to phacoemulsication ultra-sound energy is well documented(Lundberg et al. 2005; Salvi et al.2007). The purpose of our study is toexplore the combined response of CCTand CH following cataract surgeryand how this is correlated with phaco-emulsication energy. In addition, weinvestigated the changes of CH andCCT following routine clear cornealcataract surgery and assessed the effectof phaco energy on these parameters.

    Response of corneal hysteresisand central corneal thicknessfollowing clear corneal cataractsurgeryArtemios Kandarakis,1 Vasileios Soumplis,1 MichalisKarampelas,1 Ioannis Koutroumanos,1 Christos Panos,1

    Stylianos Kandarakis2 and Dimitrios Karagiannis1

    1Ophthalmiatrion Eye Hospital of Athens, Athens, Greece2Weill Cornell Medical College, New York, NY, USA

    ABSTRACTPurpose: To evaluate the effect of routine phacoemulsication in corneal viscoelas-tic properties determined by corneal hysteresis (CH) and central corneal thickness(CCT) and to explore the impact of phaco energy on the above parameters.

    Methods: Forty-one eyes of 41 patients undergoing cataract surgery were enrolledin this prospective study. CH and CCT were measured preoperatively, 1 day and1 week postoperatively. CCT measurement was performed using a non-contactoptical pachymeter followed by ocular response analyzer (ORA) examination. In-traoperatively ultrasound time, average phaco power and effective phaco time(EPT) were recorded.

    Results: Mean CH was 10.05 1.86 mmHg preoperatively, 8.25 1.85 mmHg1 day and 9.12 1.37 mmHg 1 week postoperatively (p < 0.001). The meanCCT was 534 37.33 lm preoperatively, 592.22 46.34 lm 1 day and563.21 49.84 lm 1 week postoperatively (p < 0.001). CCT and CH were sta-tistically signicantly correlated preoperatively (p = 0.01, r = 0.396). This corre-lation was not sustained on the rst postoperative day (p = 0.094, r = 0.265) andwas re-established 1 week postoperatively (p = 0.002, r = 0.568). On the rstpostoperative day, the CCT increase was positively correlated with EPT(p = 0.009, r = 0.404), which was not found between CH change and EPT.

    Conclusion: Structural corneal alterations following cataract surgery resulted in astatistical change in CH and CCT. These two parameters responded in a differentmanner that clearly demarcates their different nature. On the rst postoperativeday, CCT increase was correlated at a statistically signicant level with intraoper-ative EPT. This correlation was not found with CH reduction. Other factors,besides cornea oedema or phacoemulsication energy, could be responsible for thisCH modication.

    Key words: cataract surgery central corneal thickness corneal hysteresis effective phaco

    time phacoemulsication

    Acta Ophthalmol. 2011 The AuthorsActa Ophthalmologica 2011 Acta Ophthalmologica Scandinavica Foundation

    doi: 10.1111/j.1755-3768.2010.02078.x

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  • Patients and MethodsThis prospective, non-randomizedstudy involved patients with age-related cataract planned for routinephacoemulsication and intraocularlens (IOL) implantation. A total of 41patients, 17 men and 24 women,underwent an uneventful clear cornealcataract surgery with phacoemulsica-tion by the same experienced surgeon(AK). There were no ocular pathologyof note and no history of previousocular injury. None of these patientshad undergone ocular surgery or werecontact lens wearers. None of thepatients were suffering from glaucomaor using any topical medication.Preoperatively, a full eye examina-

    tion was performed, including visualacuity measurement, biomicroscopy ofthe anterior and posterior segmentand tonometry. CCT measurementwas rst performed using a non-con-tact optical pachymeter (HAAG-STREIT AG, Koeniz, Switzerland)followed by ORA examination. CCTand CH measurements were repeated3 and 4 times, respectively, and themean values were recorded.Mydriasis was obtained before sur-

    gery with tropicamide hydrochloride1% (Tropixal, Demo SA, Greece) andphenylephrine hydrochloride 5%(Phenylephrine, Cooper SA, Greece).After topical anaesthesia with dropsof proparacaine HCl 0.5% (Alcaine,Alcon Laboratories Hellas, Maroussi,Athens, Greece), a clear cornea 2.75-mm incision was performed. Sodiumhyaluronate (Provisc, Alcon Labora-tories Inc.) was used as viscoelasticsubstance. Phacoemulsication of thenucleus was performed with an AlconInnity unit, using phaco-chop tech-nique, followed by the aspiration ofcortical remnants and polishing of thecapsular bag. A foldable one-pieceacrylic intraocular lens (Corneal Qua-trix, Corneal, France) was implantedin the capsular bag using an injector.The viscoelastic substance was aspi-rated meticulously using an irriga-tion aspiration tip. Main incision wasleft sutureless.Intraoperatively, ultrasound time,

    average phaco power and effectivephaco time (EPT) were recorded. EPTis a measure of the total ultrasoundenergy used during the operation andwas calculated by multiplying the totalultrasound time by the average phaco

    power used. CH and CCT were alsomeasured 1 day and 1 week postoper-atively in the same manner as preop-eratively.Paired sample t-test and Pearson

    correlation analysis were used for sta-tistical analysis. Values are presentedas mean standard deviation. Alltests were 2-sided, and p values of0.05 or less were considered statisti-cally signicant.The Ethical Board of our hospital

    approved the study, and after detailedexplanation, informed consent wasobtained from each patient prior toexamination.

    ResultsForty-one eyes of 41 patients under-going clear corneal cataract surgerywere enrolled in this study. Allpatients were Caucasians, 17 weremen and 24 were women; mean agewas 73.9 7.4 years. Twenty-two

    right and 19 left eyes were operatedon and analysed. Mean CH decreasedfrom 10.05 1.86 mmHg before sur-gery to 8.25 1.85 mmHg 1 daypostoperatively, and it then slightlyrose to 9.12 1.37 mmHg 1 weekpostoperatively (Fig. 1). The meanCCT was 534 37.33 lm preopera-tively, 592.22 46.34 lm 1 day post-operatively and 563.21 49.84 lm1 week postoperatively (Fig. 2). Wedid not nd any correlation betweenage and the preoperative values ofCCT and CH (p = 0.142, p = 0.114,respectively). Mean phaco time was115.95 73.62 s, and mean averagepower was 18.35 5.22%. MeanEPT was 20.66 11.88 s.Statistically signicant differences

    between preoperative values and theones in the rst postoperative day andin 1 week postoperatively were foundfor both CH and CCT (p < 0.05).Pearson correlation analysis showed

    statistically signicant correlation

    Fig. 1. Graph showing corneal hysteresis (CH) before clear corneal cataract surgery, on the rst

    postoperative day as well as 1 week after surgery. Vertical bars: 95% condence interval.

    Fig. 2. Graph showing central corneal thickness (CCT) before surgery, on the rst postopera-

    tive day after clear corneal cataract surgery as well as 1 week after surgery. Vertical bars: 95%

    condence interval.

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  • between preoperative CH and preop-erative CCT (p = 0.01, r = 0.396).This correlation was not sustained onthe rst postoperative day (p = 0.094,r = 0.265) but it was re-established1 week postoperatively (p = 0.002,r = 0.568).On the rst postoperative day, the

    CCT increase was positively correlatedwith EPT (p = 0.009, r = 0.404)(Fig. 3). This correlation was notrecorded 1 week later (p = 0.104,r = 0.326). There was not any statisti-cally signicant correlation betweenEPT and CH changes neither on therst postoperative day nor 1 weeklater. (p = 0.689, r = )0.065 andp = 0.390, r = )0.176, respectively).In addition, on the rst postoperativeday, the corneal oedema as repre-sented with CCT increase was not cor-related with the simultaneouslydetected CH reduction (p = 0.429,r = 0.127).

    DiscussionThe introduction of CH has been asignicant step in the process of creat-

    ing a new method to determine thebiomechanical properties of the cor-nea in vivo. It has been shown thatCH is independently associated withprogressive glaucoma damage (Cong-don et al. 2006). Shah et al. (2006)suggested that CH may be helpful forlong-term monitoring of glaucomaand other diseases of the cornea whereintraocular pressure (IOP) is of criticalrole, because it provides further infor-mation about the biomechanics of thecornea, beyond that of CCT (Hageret al. 2007b).Furthermore, CH has already been

    shown to be higher in normal incomparison with keratoconic eyes(Shah et al. 2007) and two recent sep-arate studies had shown that afterLASIK surgery, CH was signicantlyreduced (Ortiz et al. 2007; Peposeet al. 2007).The aim of our study was to

    explore the effect of uncomplicatedphacoemulsication in corneal visco-elastic properties determined by CHand CCT and to evaluate the effect ofphaco energy on these two parame-ters. All measurements were obtained

    during ofce hours because it hasbeen shown that CH and CCT arealmost constant throughout the day(Laiquzzaman et al. 2006).According to the literature, CCT

    increases after clear corneal phaco-emulsication (Lundberg et al. 2005;Salvi et al. 2007). On the rst postop-erative day, we found a statisticallysignicant increase in CCT, whichremained 1 week after surgery. Themean preoperative value of CH in ourgroup (10.05 1.86 mmHg) was sim-ilar to the range of average valuesfound in literature: 9.6 mmHg in 339normal eyes (Luce 2005) and10.6 2.3 mmHg in 156 normal eyes(Hager et al. 2007b). CH demonstrateda statistically signicant decrease onthe rst postoperative day. One weekafter surgery, CH showed an ascendingcourse but remained statistically signi-cantly lower than the baseline. Compa-rable results were recently reported bytwo other studies (Hager et al. 2007a;Kucumen et al. 2008). Hager et al.(2007a) found that at day 1 after clearcorneal cataract surgery, preoperativeCH is diminished from 10.35 to 9.2. Inanother study, 1 week postoperatively,CH was also signicantly lower thanthe preoperative values showing adecrease from 10.36 to 9.65 (Kucumenet al. 2008). In this study, Kucumenet al. measured CH 3 months afterphacoemulsication and found no sta-tistically signicant difference from thepreoperative values. Based on our nd-ings and using a linear hypothesis, weestimated that if CH continued to riseat the same rate, it would reach the pre-operative value in approximately14 days following surgery (Fig. 4).Although we appreciate that linearrelationships in human tissue are rare,this nding serves only as an approxi-mation, because the actual rate of CHrecovery has not yet been dened.Previous studies (Lu et al. 2007;

    Luce 2005; Shah et al. 2006) haveinvestigated the relationship betweenCCT and CH and found that thesetwo parameters correlate with eachother. In agreement with these nd-ings, we showed that there was a posi-tive correlation preoperatively, whichceased to exist in the rst postopera-tive day. This correlation wasre-established 1 week later, althoughboth CH and CCT were still statisti-cally signicantly different from theirbaseline values. This clearly demar-

    Fig. 4. Graph showing, hypothesis based, linear ascending course of corneal hysteresis (CH)

    after the rst week.

    Fig. 3. Correlation of central corneal thickness (CCT) increases on the rst postoperative day

    and effective phaco time (EPT) (CCT1: central corneal thickness before surgery, CCT2: central

    corneal thickness 1 day after surgery).

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  • cates the different nature of these twoparameters, a difference that emergesfrom their dissimilar reaction to thesame event. In support to our nd-ings, Lu et al. (2007) measured CHduring corneal swelling induced bycontact lens wear with eye closure.During this experiment, the alterationof CCT was not associated withchanges of CH at a statistically signi-cant level. This signies that, like ourndings, CCT and CH behave in adifferent way under corneal stress con-ditions.Yao et al. (2006) showed that cor-

    neal oedema 1 day postoperativelyincreased in relation to greater EPT.Similar to this study, our results provethat CCT difference on the rst post-operative day was correlated at a sta-tistically signicant level with EPT.However, Ermiss et al. (2003) foundthat such a correlation does not exist.One possible explanation for this dis-crepancy could be that we used acohesive viscoelastic agent rather thana dispersive one.Regarding CH, we did not nd any

    statistically signicant correlationbetween EPT and CH differences. Toour knowledge, this is the rst studythat demonstrates such a result, whichin conjunction with the above-men-tioned behaviour of CCT highlights thedifferent response of those two param-eters to phacoemulsication. One pos-sible explanation for this nding couldbe that the reduction in CH is inu-enced by multiple factors beyond phac-o energy. According to previousstudies (Hager et al. 2007a; Kucumenet al. 2008), CH reduction has beenattributed to corneal oedema, whichlowers the damping capacity of the cor-nea. In our study, we found no correla-tion between corneal oedema, asrepresented by CCT increase and CHreduction. Changes of CH have alsobeen shown to occur in patients under-going pars plana vitrectomy (PPV) toan even higher extent (Hager et al.2008), although cornea was not directlymechanically affected during the PPVprocedure. Because CH has beenhypothesized to reect the global ocu-lar biomechanics and not just the cor-neas (Congdon et al. 2006; Dupps2007), the cause of this CH modica-tion could be attributed to the effect ofcataract surgery to the eye as a whole.Possible elements of this effect could be

    the clear corneal incision or themechanical stress of phacoemulsica-tion on the globe.In conclusion, structural corneal

    alterations after clear cornea cataractsurgery lead to a compromise of itsbiomechanical properties as demon-strated by a statistically signicantreduction in CH. CCT and CHrespond in a different manner aftercataract surgery. Other factors, besidescorneal oedema or phacoemulsicationenergy, could be responsible for thisCH modication. Further studiesshould be undertaken to elucidate thetime period where CH returns to itspreoperative values as well as the fac-tors that inuence this behaviour.

    AcknowledgementThere has been no nancial support(grants) for this study. None of theauthors hav...

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